The inhibition by three essential oils of the growth and esterase production of German cockroaches, Blattella germanica (L.) (Blattodea: Ectobiidae), with and without symbiotic bacteria, was determined in laboratory assays. Essential oils extracted from Lilium brownii var. viridulum Baker, Croton tiglium L., and Lonicera japonica Thunberg were mixed with a powdered rat food at a 10% concentration and fed to cockroach nymphs. Body mass and body length were measured in adults in all three treatments. Acetylcholinesterase and nonspecific esterase activities were compared between the Lilium and Croton essential oil treatments. The nonlethal doses of these essential oils reduced cockroach growth, while the growth of the females lacking symbiotic bacteria was significantly reduced when fed the diet containing Lilium essential oil. Lilium and Croton essential oils significantly inhibited esterase activity. Our results indicate that symbiotic gut bacteria might be involved in mediating detoxification in the German cockroach.